To obtain optimal performance of a composite reinforced with fibers in the form of rovings or fabrics, the lay up of the fibers is essential. Unintentional movement of fibers resulting in wrinkles or displacement of the fibers can have a severe effect on the final laminate properties, and can result in costly repairs or discarding of the laminate.
Especially in large molds, where a large amount of fiber reinforcement is applied in a predefined sequence, the risk of local displacement of fibers is high, unless new layers are continuously fixated. Also, if movement of the mold or inserts into the mold is necessary, the risk of displacement is high.
Currently, to ensure proper placement of mainly fabrics, these are often stitched together for each new layer to form a more stable package of fibers. This is however very labour intensive and in some cases, the stitching of fabrics can be very difficult or impossible to perform, as it requires the fabrics to be lifted at least partly for proper use of the stitching equipment. Furthermore, the stitching negatively affects the fiber structure of the fabrics and thereby induces suboptimal material performance.
Alternatively, a holding device for fabrics can be positioned at the side of the mold, which can then be used to hold each new layer of fabric. This often requires use of excess fabric which should subsequently be cut away and can to some extent also result in a disruption of the fiber structure. This holding method can only be used around the edges of the mold.
The use of adhesives for attaching purposes is limited, as currently available adhesives can interfere with the subsequent wetting of the fibers in the area covered by the adhesive, and will also affect the material properties negatively due to inferior compatibility between adhesive and the thermoset material used to produce the composite part.
The known reactive hot melt adhesives can be used for a variety of purposes and are often based mainly on polyurethanes reacting with moisture in the air, but also adhesives based on ethyl vinyl acetate copolymers. Such adhesives can not be incorporated in a subsequently applied thermosetting resin.
In U.S. Pat. No. 7,271,202 B2 a reactive hot melt composition is disclosed, which has excellent adhesion and curing properties. The composition comprises in ethylene-acrylic acid copolymer, and ethylene-methacrylic acid copolymer, and/or an ethylene-acrylic acid methacrylic acid terpolymer along with free radical cross linking initiators.
An adhesive tape which can be used to adhere fiber fabrics to a mold or other fiber fabrics is produced by Saertex and commercially available under the trade name Saerfix. This tape can also be pre-applied to the fabric from the manufacturer. As in the disclosed invention, this tape is also incorporated into the thermosetting resin matrix during fiber wet out without negative effects on the mechanical properties.
Such tapes do however suffer from some limitations. The geometry of the adhesion surface is limited to simple geometries, which must be cut to shape. As the tapes have adhesive surfaces, until use, the surface must be covered by a carrier material, which must be removed before application and discarded. The shelf life is somewhat limited. Such tapes are relatively expensive. Automated lay-up of such tapes will be rather difficult.
In US 2010/0029155 A1 adhesive coated fabrics are disclosed, which are suitable for use in composite materials. The adhesive coated fabrics are coated on at least one outwards facing surface with an acrylate adhesive. In US 2007/0023975 A1 methods of making fiber reinforced molded articles and fiber mats using anaerobic binders are described.